Another strange event

[Oliver]

Here's another starange event. This occured in the middle of a sequence not long after a series of system suitability runs.
The info I can supply is that 1st of all these are negative peaks turned upright by using the negative polarity function in the detector.
2nd the mobile phase in a nitric acid solution, which has greater absorbance than the samples (that's how the negative peaks arise)
3rd the samples are in water only (not nitric) that is probably the initial peak up to 200mV seen just after the injection peak.
The starnge thing is that that sometimes part or all of this disappears. also the standard peak is affected and a new peak is observed:!:
the first and last chromatograms were fine. Both the 'water' and standard peaks are of the expected areas.
Standard A is from one vial, Standard B is a separate preparation but injections are all from the same vial.
Any ideas appreciated. Thanks for reading.

Standard A run 1 (good)


Standard B run 1 (bad)


Standard B run 2 (bad)


Standard B run 3 (bad)


Standard B run 4 (good)

[unmgvar]

for this case Olivier i think that you might have a late eluting peak from earlier injections that gets into your next chromatograms.
it is late right now in my time zone so, try finding a pattern between your 3 peaks in correletion with the run time, and the amount of time it takes your sampler to inject.
you might find a pattern that will help you set a longer correct run time.
if not successfull, wash your system and coulumn and inject one time for 20-15 minutes and see what happens. it should give you a general indication if my hunch is correct or not.

[unmgvar]

olivier,

looking again at your peaks, i see that also they are fronting and look pretty ugly for rt 3 and 4.5. see if you need to change your sample solvent's.
are you using the correct buffer at the right PH for your aplication?

[Oliver]

Yes they do indeed front a lot and the greater the concentration of sample injected the peak "triangle" starts from the same RT but ends later.
It is charachteristic of this product. The diluent is water and the mobile phase is a dilute solution of nitric acid in water.
The only difference is that the water used for the mobile phase is double deionised but the water used in sample preparation is purified water (1st stage RO, followed by deioniser)

[syx]

What kind of chromatography did you perform? You filled the system with water entirely…

[KarenJ]

Hi,

Since you are making multiple injections, the poor reproducibility that you are seeing from one injection to another may be caused by your vial. If the seal on your vial is too tight, you can actually create a small vacuum inside the vial which causes the injector to have difficulty drawing up the right volume for subsequent injections. This is most likely if you are using screw-cap vials. We have had so much trouble with this that we only use snap-cap vials now.

Hope this helps.

KarenJ

[Oliver]

Syx: yes it is nitric acid in water, it is an anion exchange column.
KarenJ: i thought of that as well and loosened the caps but there was no change.

[unmgvar]

Olivier,

have you tried longer run times for this aplication?

for the fronting i would first direct you to the LCGC lc troubleshooting article of december 2002, page 3. maybe the information there can help you.

[Oliver]

unmgvar; yes we extended the runs but no other peaks came out. Regarding the fronting, I don't have any experience on anion exchange chromatography but any way since we're tied with validated methods and cannot change them (no time and resources to re-validate) unfortunately we're stuck with that.

[syx]

I do not have any experience with this kind of chromatography. Could anyone explain the cause of fronting peak in ion-chromatography? Is the reason same as RP-chromatography?

[JM]

Hello! oliver,
it seems your extra peak generation is related to the unretained peak at about 1.2 min. I would like to know what is this peak ? In Ion exchange chromatography normally the opposite ion of ionic compund apears at this RT such as Na, K etc. It looks like that your sample when ionised completely gives you one peak and if partly ionised ( indicated by intensity of unretained peak) gives you two peaks and this is quite possible.

I would recommend to prepare sample in mobile phase and see have you changed the injection volumn? The ionisation vis-a-vis separation is also dependent of Column saturation ( in exchange chromatography it takes long time ).

JM